Scientists Go to Great Heights to Understand Changes in Earth’s Atmosphere

Human activities—from growing rice and burning coal or wood, to driving cars and testing nuclear missiles—have impacted the Earth’s atmosphere over time. Cleansing the Earth’s environment is of growing interest in the new era of humanity, unofficially called the Anthropocene epoch. To better understand the impact of the human biogeochemical footprint on Earth, scientists at the University of California San Diego are literally climbing mountains to study the planet’s sulfur cycle—an agent in cardiovascular fitness and other human health benefits and resources.

UC San Diego researchers Mark Thiemens and Mang Lin, a recipient of the 2018 Chancellor’s Dissertation Medal, along with their colleagues from the Chinese Academy of Sciences, the Institute of Tibetan Plateau Research and the Research Center for Environmental Changes, bundled up and trekked up snowy slopes in the region to collect and analyze data around the sulfur isotope composition of sedimentary sulfates from a lake bed near Mt. Everest. Their new research findings are published in an article titled “Atmospheric sulfur isotopic anomalies recorded at Mt. Everest across the Anthropocene” in the June 18 issue of PNAS. The study provides a broader view of the origin of sulfur isotopic changes over the past two centuries and deepens insights into the Earth’s sulfur cycle during the evolution of early life.

The Earth’s sulfur cycle contains both atmospheric and terrestrial processes. On land, the cycle begins when rocks weather and release stored sulfur into the air. There, it converts to sulfate, which is absorbed by plants and microorganisms. Animals, humans included, consume these organic forms. As living organisms die and decompose, some of the sulfur is released again, entering the tissues of microorganisms, propagating the food chain. Sulfur is also released in nature by volcanic eruptions, water evaporation and the breakdown of organic matter in swamps and tidewaters. An atmospheric agent on the move, sulfur transports through elements like dust, reaching the glaciers where it gets stored. Heat-absorbing agents cause ice to melt faster and loss of a basic human resource—water.

Thiemens, a distinguished professor of chemistry and former dean of the Division of Physical Sciences, conducts research in the three Polar regions—Antarctica, Greenland and the Tibetan Himalayas, termed “The Third Pole”—where a sulfur cycle record is lacking. Data from this area could represent as many as two billion people, and it could lead to better understanding of how to counteract more heat-absorbing agents transporting to the ice and causing faster melting and water loss. This is significant since nearly half the world’s population obtains water from the Himalayas.

Sunlight atop Mt. Everest. Photo by Mang Lin

Conducting research amidst the harsh climate and altitude of the Himalayas and the Tibetan Plateau gave the scientists access to a well-preserved record of air particulates, specifically the changes in the Earth’s sulfur cycle during the most recent centuries. Importantly, the scientists observed the change in the atmosphere prior to the Industrial Revolution. The research site—a freshwater, Alpine lake—was chosen for its remoteness at Mt. Everest, the altitude and the fact that lakes are natural curators of atmospheric aerosol history. The scientists extracted core samples from the lake bed, transported them back to UC San Diego’s Urey Hall, and used a table isotope spectrometer and radioactive natural clock to measure the samples.

“In summary, our measurements of multiple stable sulfur and lead isotopes, major, trace and rare earth elements in a two-century Himalayan sediment record suggest that the observed changes in sulfur cycling at the second industrial revolution reflects more dust-associated sulfates and climate-induced weathering/erosion affecting the Himalayas in the last century than the 19th century,” said Lin.

Thiemens added that the specifics obtained from this heavily populated region with the isotope ratio mass spectrometer help sharpen science’s diagnostic toolkit, which can be applied anywhere in the world. Additionally, the work of the research team supplements the time record, as well as helps to develop new techniques that allow a deeper understanding of where chemical species come from, how they change and interact over time, and insight into what has happened over more than 100 years.

“The species are also involved as a player in global warming, and its interactions are quite complex and variable. This gives us a better picture of what happens in this part of the world,” noted Thiemens. “We are also helping to predict where we are heading.”

This study was partially funded by the National Natural Science Foundation of China (grants 41630754 and 41721091) and supported by fellowships from the Guangzhou Elite Project (JY201303) and the visiting scholar program of Academia Sinica.

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